Note: Descriptions are shown in the official language in which they were submitted.
Z(~ 8
POLYHYDRIC PHENObS AS CHAIN ExrENDER$
EO~ C~RTAIN RIS~ALEIMIDE RESINS
BA~<GROUND OF THE INVENT ON
In recent years po~imides have had increasing use as therrnosetting
resins in high performance applications, as the rnatrix resin for reinforced com-
posites in spacecraft and missiles and for syn~actic foams, as well as for lami-nates in printed circuit boards and other electronic applications. When poly-
imide resins are cured they generally afford a polymer with a high glass transi-o ffon temperature and excellent chemical (environmental) stability with particu-
larly good resistance to moisture and to oxidative degradation at elevated
temperatures. However, cured resins typically are extensively crosslinked
leading to products which are very brittle, that is, having low fracture toughness.
Many bismaleimides manifest the unfortunate property of beginning to
15 polymerize at a temperature which is at or Just above the melting point of the
monomer, that is, the temperature dfflerential between meltin~ and onset of
polymerization is small. As a result it is difficult to maintain the uncured resin in
a fluid state, and th0 accompanying difficulty in attain.ng a homogeneous melt
leads to well documented processing difficulties. The patentee of U.S.
20 4,464,520 addressed this problem and providad a class of bismaleimides
(BMls) with increased pot life, therefore a Rlarger processing window~. How-
ever, ths compositions taught there still afforded cured polymers which were
brittle.
Becauss the brittleness of the cured product arises from extensive
25 crosslinking during polymerizatlon, many efforts have been rnade to reduce the
crosslink density in the cured product to afford toughened BMls without ad-
versely impin~ing on other desirable properties. See H. D. Stenzenberger et al.,l9th International SAMPE Technical Conference, October 13-15, 1987, pages
372~5. One ~eneral approach has been to react BMI monomers with certain
30 reactive bifunctional reagents having active hydrogens to afford Michael addi-
tion products. This reaction and the resulting Michael adduct may be exempli-
fied using a diamine as the reactive bifunctional reagent by the equation,
.
. .~ . .
30~018
NH2~ NH2 J~ ~ o~o
As the foregoing equation shows Michael add tion reduces double bond density
in the BMI monomer (or oligomer) resui~ng in a lower crosslink densi~y in the
cured product. The diarnine also can be viewed as a chain extender in addition
to its function of reducing crosslinkin~ density.
o Michael addition aenerally is a bæe~talyzed r~action, and since
amines as bases seNe as their own catalysts this is one reason why amines
usually are quite reactive in Michael addition. Where alcohols are used, the re-action with nitrogen-substituted maleimides requires a base catalyst as an addi-tional component; A. Renner et al., Heh~. Chlm. Acta., 61, 1443 (1978). These
workers also have given the sole instance of the reaction via Michael addition of
a polyhydric phenol (bisphenol A) to a typical BMI monomer, with chain ex-
tension requiring a discrete base catalyst. However, the use of a third compo-
nent as a catalyst along with a chain extender polyhydric phenol ~enerally is
undesirabls sinc~ the resulting product retains the catalyst as a component
2 0 which might sign-~icantly degrade the perfonnance of the final cured resin. The
necessity of using basic catalysts for chain extension with polyols is particularly
unfortunate, since a significant advantage of polyols i5 that they are non-car-
cinogenic whereas aromatic diamines used as chain extenders often are car-
cinogenic.
The bismaleimides of U.S. 4,464,520, representative of which is the
structure
~ S--CH2CH2-S~
3 o 0~ N ~0 Oq, N ~o
35 could be expected to undergo Michael addition sluggishly, n at all, because of
the relatively hindered naturs of the maleimids double bond. Quite unexpect-
. .
, ": . :
- ~ : .
3 ~ 0~8
edly it was found that not only did sueh materials undergo Michael addition, butin faet they reacted facilely with the less r0active polyhydrie phenols. But notonly did ths polyhydric phsnols readily r~act with the aforementioned BMls,
they did so in an uncatalyzed reaction, that is, in the absence of a base catalyst.
5 This totally unexpected behavior afford0d cured r~sins eontaining no perfor-
mance-degrading eomponents and led us to examine some r~levant perfor-
manee characteristics of representative ehain-~xt~nded eured resins. We have
found that relative to the parent eured resin, ehain extension generally has re-duced brittleness and improved the toughnsss of the eured resin, with the latter10 having a superior dieleetrie eonstant and loss factors and eomparable coeffi-cients of thermal expansion and chemical resistance. Most surprisingly, the
chain-extended BMls have wider processing windows than either the parent or
diamine chain-extended BMI resins.
SUMMARY OF THE INVENTION
The purpose of this invention is to prepare bismaleimide resins chain
extended with polyhydrie phenols in the absence of a eatalyst. An embodiment
eomprises the reaetion of eertain di-ortho-substituted BMls, as exemplified by
1,2-bis(2-maleimidophenylthio)ethane, with polyhydric phenols in the absence
of any third component as catalyst. In a more specific embodiment the poly-
2 o hydrie phenol is a dihydrie phenol. In a still more specific embodiment the dihy-
drie phenol is hsxafluorobisphenol A. Another embodiment is a thermosetting
resin which is the chain-extended reaction product of eertain di-ortho-substi-
tuted bismaleimides such as 1,2-bis(2-maleimidophenylthio)ethane with poly-
hydrie phenols and whieh eontains no third eomponent. Yet another embodi-
25 ment is the eured resin resulting from thermal treatment of the preeeding ther;mosetting resin. Other embodiments will become elear from the ensuing de-
scription.
DESCptlPTlO~LOF THE INVENTION
Our invention arises from the unprecedented observation that a class of
30 di-ortho-substituted BMls undergoes Michael addition with polyhydric phenols
in a reaction uncatalyzed by any third component and in the absence of any
base cata~st to yield ehain-extended bismaleimides as reaction products. The
chain-extended BMls are therrnosetting resins having an extended pot life and
therefore having an increased processing window relative to non-extended
~ . ............. . - . . . :
. . . . . . .. . . . .
.... . ,. ~ -
. ~ . ~ . ... ~ .
4 ~ 018
BMls. The polym~rs from the fully cured chain~xtendcd resins have not only
improved fracture toughness, but also have a more ~avorable dielectric constant
and dielectric loss factor, neither of which are predic~able.
The BMI monomers used in the practice ~f our invention are thsse
5 taught in U.S. 4,464,520 and which have the formula
.
~ Rb ~:
10Ra~X--(CH2); -X~R,
0~0 0~0 . :,
where Ra and Rb each independently is hydrogen, an alkyl or alkoxy group
containing up to 4 carbon atoms, chlorine, or bromine, or Ra and Rb together
form a fused 6-membered hydrocarbon aromatic ring, wi~h ths proviso that Ra
and Rb are not t-butyl or t-butoxy, where X is 0, S, or Sc, i is 1-3, and the alkyl-
20 ene bridging group is optionally substituted by 1-3 methyl groups or by fluorine.
In a preferred embodiment Ra = Rb = H, especially where X = S, and par-
ticularly where X = S and i = 2. In th~ most favored embodiment the bis-
maleimideis 1,2-bis(2-maleimidophenylthio)ethane.
The bismaleirnide monomers are reacted in the absence of a third com-
25 ponent as a catalyst via Michael addition with a polyhydric phenol acting as a
chain extender. By "polyhydric phenol~ is meant a compound having at least 1
aromatic ring and having at laast 2 hydroxyl groups attached to the aromati~
ring(s) in the compound. The most important class of polyhydric phenols is that
of dihydric phenols, and within this class tha subset of broadest availability is
30 that of th~ resorcinols, i.e., 1~3-dlhydroxybenzenfls optionally substituted with
one or more alkyl groups on the aromatic ring, partlcularly where the alkyl
group contains from 1 through about 6 carbon atoms. Examples include
2-methylresorcinol, 4-methylresorcinol, 5-methylresorcinol 6-methylresorcinol,
4-ethylresorcinol, 4-propylresorcinol, 5-pentylresorcinol, 5-hexylresorcinol,
3 5 2,4-dimethylresorcinol, 2,5-dimethylresorcinol, 4,5-dimethylresorcinol, 4,6-di-
methylresorcinol, and so forth.
..
.
..
- ~ ~ . . . .
`` 5 ;~ )18
Although the 1,3-dihydroxybenzenes may be the most widely available
class of dihydroxybenzenes, nonetheless the 1 ,2~ihydroxybenzenes
(pyrocatechols) and 1,4-dihydroxybsnzenes (hydroquinones) also are suitable
dihydric phenols in the practice of this invention. Illustrative examples include
5 pyrocatechol, 3-methylpyrocatechol, ~methylpyrocatechol, the ethylpyrocate-
chols, propylpyrocatechols, butylpyrocatechols, pentylpyrocatechols, hexyl-
pyrocatechols, hydroquinone, the aikyl-substituted hydroquinones where the
alkyl group contains from 1 through 6 carbon atoms, the dialicyl-substituted hy-droquinones, and so on.
lC Another class of dihydric phenols is that of the dihydroxynaphthalenes
where the hydroxyl groups may be on the sama or on dfflerent rings. Illustrativemembers of this class include 1,2-dihydroxynaphthalene, 1,3-dihydroxy-naph-
thalene, 1,4-dihydroxynaphthalene, 1,5-dihydroxynaphthalene, 1,6-di-hydroxy-
naphthalene, 1 ,7-dihydroxynaphthalene, 1 ,8-dihydroxynaphthalene,
2,3-dihydroxynaphthalene, 2,5-dihydroxynaphthalene, 2,6-dihydroxy-naphtha-
lene, etc.
Another important class of dihydric phenols used in this invention is that
given by the formula
ZO HO~_ ~OH
25 where Y = (bond), CH2, C=O, C(CH3)2, C(CF3)2, O, S, S02, SO, and wherë
Rc, Rd are hydrogen or alkyl groups containing from 1 through 6 carbon atoms.
An important subgroup is that where X 2 (bond), that is, where the 2 aromatl'c
rings are directiy pined. Members of this class are illustrated by
4,4'-dihydroxydiphenyl, 3,3'-dihydroxydiphenyl, and similar dihydroxydiphenyls.
30 Another important subgroup is that where X = C(CH3)2 or C(CF3)2. In the
simplest case, where Rc = Rb = H, such materials are commonly referred to
as bisphenol A and hexafluorobisphenol A. Other members of the group en-
compassed by the aforegoing formula include 4,4'-thiodiphenol, bis-
(hydroxyphenyl)ether, bis(hydroxyphenyl)sulfoxide, bis(hydroxyphenyl)sulfone,
35 and bis(hydroxyphenyl)methane.
,
.. : . . - .
. . . .
- .
- - . . .
.
.
. . . . .
; . . .
2~G~0~8
Among the phenols which are st least trihydric may be mentioned
tetraphenolethane (1~1~2~2 tetrakisQlydroxyph~ny~thane)~ 1,1,1-tris(hydroxy-
phenyl)ethane, tris~ydroxyphenyl)methane~ tetrakis~ydroxyphenyl)methane,
1,3,5-tris(hydroxyphcnyl)benz~ne, 1,3,5-trihydroxybenz0ne. and the phenol-
5 formaldehyde condensation products commonly known in the trada as No-
volacs and having the formula l ~ -
1C [~ ]t
The polyhydric phenol will be used in a molar proportion relative to the
bismaleimide as little as about 0.05 and as great as about 2. That is, the molarratio of BMI to polyhydric phenol used in the preparation of the chain-extended
BMI may be as great as 20:1 or as little as 0.5:1. Quits often little change is
15 seen in the resulting product when less than about 0.1 molar proportion of
polyhydric phenol is employed, and the glass transition temperature often is ad-versely affected when more than 1 molar proportion is employed. Conse-
quently a molar proportion of polyhydric phenol relative to BMI which is pre-
ferred in the practics of our invention is from about 0.1 to about 1Ø
The chain extension reaction is carried out rather simply. The bis-
maleimide and polyhydric phenol are mixed in a molar proportion from about
20:1 to about 1:1 and are reacted generally in a fluid melt state to achieve
homogensty. A temperature between about 160-170C ordinarily will suffice,
although an even lower temperature may be adequate where a fluid melt state
25 can be achieved. When the reaction is complete the mixture is allowed to cooland solidify to afford a near quantitative yield of chain~xtsnded bismaleimides.The resulting chain-extended BMls be~in to undergo thermal polymer;
kation in the range from abou~ 170C up to at least 250C. However, poly-
merization peaks at a temperature between about 250C up to at least 320C.
30 Thermal curing is perhaps most preferably done in an inert atmosphere, such
as nitrogen.
As the data in the following examples will show, the dielectric constant
and loss factors for our chain-extended BMls are superior to those of either thenon-extended BMI or to a typical diamine chain-extended BMI. The coefficient
35 of thermal expansion of all of our cured chain-extended BMls are comparable to
the parent or diamine chain-extended BMls, as are water and methylene chlo-
. ., ,
.. ~ . . -
- ,. , . .- ~
:. . .: : . . .
7 ~ 8
rids absorption properties. ~exural data show ~hat polyhydric phsnol chain
extension has reduced brittleness and improved toughness of the cured resin
over that of the cured non-extended BMI. The chain-~xtended BMls of our in-
vention also haYe broader processing windows than ~eir diamine chain-ex-
5 tended counterparts, exhibiting a dfflcrence of at least 100C between theirmeiting point and the onset of thermal polymerization. In summary, we have
demonstrated chain extension with polyhydric phenols produces BMI resins
having properties which are comparable or superior to those of diarr ine chain-
~xtended counterparts, but without the complications of aromatic diamine
o toxicity and carcinogenicity.
EXAMPLES
The following list is of abbreviations used throughout this section.
BPA = bisphenol A
6FBPA = hexafluorobisphenol A
TDP = 4,4'-thiodiphenol
PG e phloroglucinol
TPE = 1,1,2,2,-tetrakis(hydroxyphenyl)ethane
2 o APO-BMI = 2,2'-bis(2-maleimidophenylthio)ethane
Chain-Extension of APO-BMI with Dlols. The following example illus-
trates the general procedure employed for the chain-extension of APO-BMI with
diols. A 2-liter resin kettle was fnted with a reflux condenser, mechanical stirrer,
25 N2 inlet, drying tube, thermocouple, and a heating mantle. Under a slight posi-
tive N2 flow, the empty kettle was preheated to 120C, then a mixture of'
435.1 9 of APO-BMI and 64.9 9 of 8PA (mole ratio 3.5:1) was added over the
course of 18 minutes, while maintaining mechanical stirring to facilitate melting.
The fluid melt was maintained at 160-170C for a period of 2 hours while being
30 stirred under N2. The clear homogeneous reaction melt was poured into an
enameled steel tray, and allowed to cool and solidify. The yield of resin was
>95%.
Using the same procedure, reagents and proportions were varied to
produce the various AP0-BMI/BPA, APO-BMI/TDP, and APO-BMI/6FBPA
3 5 systems cited.
. .
... . , .
.
. . , . ., :
., -
.
- . ~ . ~. . . ~
: - . . . . . ~ :
- -
Curin~ Dlol Chain-Extended APO-BMI R~sins. Small (5-10 9) sam-
ples of diol chain-extended APO-BMI resins were weighsd into 57 mm diameter
aluminum weighing dishes. The samples wer~ placed into N2-purged ovens,
and heated to 175C; fluid melts r0sult~d. Th~ sarnples wer~ cured at 175C
5 under N2 for 24 hours, then the temperature was incrcæcd to 240C, and th2
samples were cured at this t~mperature under N2 for an additional 24 hours. It
is recognked that these curing Umes are excessive, and that shorter cure times
may be employed.
Chain~ nslon and Curlng of APO-BMI with Polyols. Because PG
lO and Ti'E are poiyfunctional, it was anticipated that ~ey would quickly yield in-
fusible uosslinked gels. Therefore, chain-~xtension and curing were combined
in a single step. Observation of the melt behavior of thes~ systems suggests
that the two steps could have been conducted separately, however. The fol-
lowing procedure, describing chain-extension with TPE, is general.
APO-BMI (6.7 9) and TPE (1.0 9), mole ratio 3.1:1, were mixed in a
B7 mm diameter aluminum weighing dish. The mixture was placed into a N2-
purged oven, and heating was begun. At a temperature of 130C, the mixture
began to melt, and at 165C a fluid, clear melt was obtained. After 1 hour at
180C, the resin was still a free-flowing melt. The resin was cured at 180-190C20 for 18 hours. The temperature was raised to 240C under N2 for 8-1/2 hours.
It is recognized that these cure times are excessive and that shorter cure Umes
may be empioyed.
Thermal Analyses. Both DSC (differential scanning calorimetry) and
TGA (thermogravimetric analysis) were performed using a DuPont Model 9900
2!!i Thermal Analysis system. DSC analyses of uncured resins were conducted at
~ T=5C/min under N2, and cured resins were analyzed at ~T=10C/min
under N2. All TGA analyses were conducted at ~T=10C/min in air. CoeffP~
cients of therrnal expansion (CTE) were determined using a Mettler TA-3000
Thermal Mechanical Analysis system.
Electrlcal Analysls. Dielectric constants (~ and loss factors (tan~)
were determined using a Digibridge system at 1MHz and 23C. Samples were
preequilibrated at 0% and 50% relative humidity prior to testing.
Mold Curlng. Resin formulations were placed into beakers, which were
then placed into vacuum ovens purged with N2. The samples were heated to
150-160C to give fluid melts. The melts were degassed under vacuum at
160C for 30-60 minutes. Vacuum was released and was replaced by a N2
: - - . ,. ~ . .~ , . . .
. .
9 ;~ ~D~O~L8
purge, and the degassed melted r~sins were pourad into silicone rubber flexural
modulus molds. The resin-filled molds were placed into an N7-purged oven
which was preheated to 175C. The sarnples were cured at 175C for 24
hours. The sarnples were removed from the molds, and were further cured
5 free-standing at 240C under N2 for 24 hours. Samples were allowed to cool to
room temperature slowly to prevent uacking.
Flexural Propertles. ~exur~ propertiss of cured APO-BMI and AP0-
BMI/BPA (mole ratio 3.5:1) were deterrnined by the 4 point bend test at room
t~mperature following ASTM-D790. A loading span of 1.016~, and a support
o span of 2.032~ wer~ used.
Water Uptake. Sarnples of cured resins were weighed before and after
being suspended in a large excess of reflwdng distilled water for 24 hours.
Methylene Chlorlde Uptake. Samples of cured resins were weighed
before and after being suspended in a large excess of CH2C12 maintained at
15 room temperature for 72 hours.
Tables 1~ summarize some salient characteristics of chain sxtended
resins and the cured resins therefrom.
.. . . .............. ~ . . ................................ .
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N ~ N ,~ o o o o C:
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O~O ~ :
NO ~ _ 3
Ll & __~___
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JJ~ ~1 ~ ~ ~1
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c~ ai ,
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